The invention is directed to medical devices implantable into the body of a living being having detector means with variable sensitivity for detecting events occurring with respect to a physiological function. The invention is also directed to a method for the operation of such a device.
The term "sensitivity" as used herein is to be understood as a threshold that a signal parameter or signal parameters of a signal corresponding to the physiological function that is detected, for example the amplitude and/or the steepness of an electrical signal, must exceed in order to lead to a detection. A high sensitivity therefor corresponds to a low threshold, whereas a low sensitivity corresponds to a high threshold.
The term "stimulation intensity" is used comprehensively herein and includes, singly or in combination, the duration, frequency, repetition rate, amplitude, etc., with which means for stimulating are activated.
One problem that arises in prior art devices to which the invention relates is that while one would like to set the sensitivity optimally high in order to guarantee that all occurring events are in fact detected, on the other hand, the sensitivity cannot be set too high without risking that the detector means erroneously detects noises superimposed on the signal corresponding to the physiological function as events. In practice, the sensitivity is therefore usually set to a value that represents a compromise.
A device of the type initially described that is fashioned as a heart pacemaker is disclosed in the publication "Pulse Generator 704--Physician's Manual", produced by Siemens-Elema AB, Solna, Sweden. The publication is dated March 1985.
The disclosed heart pacemaker provides what is referred to as an intracardial electrocardiogram (IEKG) signal that is supplied to the detector means of the heart pacemaker. The IEKG signal is displayed in conjunction with registration means.
When a corresponding function of the heart pacemaker is activated, those events that the detector means of the heart pacemaker detects as natural heartbeats are marked in a registration of this signal. An attending physician is thus in a position to initially set the detector means to a sensitivity at which all events in the registered signal, that the physician recognizes as natural heartbeats, are detected.
Subsequently, the attending physician sets the sensitivity of the detector means to a value that is higher by a safety margin, this value being selected depending on the individual conditions of the patient. Since, after the end of a stay in a clinic in connection with the implantation of a pacemaker, the patient only sees the attending physician at great intervals, there is the risk that conditions can arise that no longer correspond to the adjustment of the sensitivity of the detector means that was selected by the attending physician. For example, the position of the endocardial electrode that conducts signals corresponding to the electrical activity of the heart to the detector means can change. This can lead to malfunctions, regardless of whether the existing adjustment of the sensitivity is too high or low for the altered conditions. In the former case, there is the risk that noise signals will be erroneously detected as natural heartbeats and, consequently, a stimulation of the heart that is required per se is omitted. In the latter case, there is the risk that all natural heartbeats that occur will not be detected and, consequently, the heart of the patient will be unnecessarily stimulated. This is just as undesirable since the well-being of the patient will be jeopardized.